A number of efforts are currently made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices. Deep-ultraviolet lithography using KrF or ArF excimer laser has become the main stream of microfabrication technology. The deep-UV lithography combined with chemically amplified resist is capable of patterning to a feature size of 0.2 μm or less while pattern processing to a feature size of less than 0.065 μm now becomes the target. Also in the electron beam (EB) lithography, the progress of chemically amplified resist has reached a practically acceptable sensitivity to EB of higher energy, indicating possible processing to a finer size. Further in the lithography using EUV, use of chemically amplified resist is thought essential to achieve a practically acceptable sensitivity.
In the course of development of such chemically amplified positive resist compositions, the addition and modification of various resist components have been proposed in order to ameliorate the outstanding problems of resolution, sensitivity, pattern profile, post-exposure delay (PED, a change of pattern profile with standing time following exposure) and substrate dependency. Among others, the solvent is an important component to impart a uniform coating capability to coating compositions including chemically amplified resist compositions. A variety of solvents have been proposed to enable effective resist coating. An ability to form a uniform coating is indispensable to improve line edge roughness (LER) and resolution. To attain the goals of homogeneous dissolution of resist components and deposition of a uniform coating at the same time, a mixture of solvents is generally used rather than individual solvents.
One exemplary solvent mixture is proposed in JP-A 2000-267269 as comprising propylene glycol monoalkyl ether acetate and propylene glycol monoalkyl ether. This solvent mixture is useful in inhibiting formation of defects in the resist film and becomes effective when a proportion of propylene glycol monoalkyl ether exceeds 50% by weight based on the total solvent weight.
Another solvent mixture which is effective for improving LER is proposed in JP-A 2001-183837 as comprising propylene glycol monoalkyl ether acetate, propylene glycol monoalkyl ether and optionally, γ-butyrolactone. Allegedly a choice of this solvent mixture overcomes the problem of micro-grains (granular foreign matter of 100 μm or smaller) during development.
A further solvent mixture which provides a resist composition with storage stability and a good pattern profile is proposed in JP-A H07-084359 as comprising propylene glycol monoalkyl ether acetate, propylene glycol monoalkyl ether, and ethyl lactate. The solvent mixture becomes effective when a proportion of ethyl lactate is 30 to 90% by weight based on the total solvent weight.
In the foregoing proposals, line edge roughness (LER) is improved by a modification of the chemically amplified positive resist composition. LER may also be improved by a modification of the pattern forming process. The method of JP-A 2005-19969, for example, intends to improve LER by feeding a solvent gas to a resist pattern resulting from development for dissolving the resist surface and then effecting heat treatment to evaporate off the solvent and to sinter the resist film. This method, however, is undesirable in that a unit for feeding a solvent gas to the resist pattern after development and a unit for recovering the solvent must be added to the developing system, and so the overall system becomes costly.